1. Technical Field
The present invention relates to a fusing device for fixing an image on a sheet, and to an image forming apparatus including the same.
2. Description of Related Art
In resent years, image forming apparatuses of an electrophotographic system (hereinafter simply referred to as “image forming apparatus”) have been widely used mainly in offices and the like. The image forming apparatus includes a photoreceptor, a charging means, an exposing means, a developing means, a transfer means and a fixing means for performing charging, exposing, developing, transferring and fixing procedures to form an image on a sheet-like recording medium (hereinafter also simply referred to as “sheet”).
As a fixing means, a fusing device of a heat roller fusing system is used for example. The fusing device includes a fuser roller and a pressure roller. The fuser roller and pressure roller form a pair of rollers that are pressed to be in contact with each other. A thermal source such as a halogen heater is arranged as a heating means inside at least one of the fuser roller and pressure roller.
In the fixing procedure, the thermal source heats the pair of rollers to a predetermined temperature required for fusing (hereinafter referred to as “fusing temperature”), and then a recording medium on which a to-be-fixed toner image is formed is fed to a fuser nip which is a portion where the fuser roller and pressure roller are pressed to be in contact with each other. The toner image passing through the fuser nip is fused by the heat transferred from at least one of the fuser roller and pressure roller and is fixed to a recording medium such as a sheet of paper by a pressure from the fuser roller and pressure roller. If the time required for the thermal source to raise the temperature of the fusing device to a fusing temperature (hereinafter referred to as “warm-up time”) is long, it takes time to form an image on the recording medium. A shorter warm-up time would improve the user's convenience.
The fusing device attached to the image forming apparatus capable of full color printing uses a fuser roller having a surface provided with an elastic layer formed of, for example, silicone rubber. Such a fuser roller causes the elastic layer on the surface of the fuser roller to be elastically deformed to correspond with the irregularity of the to-be-fixed toner image, making the fuser roller in contact with and cover up the to-be-fixed toner image. This allows a multicolor to-be-fixed toner image having a larger amount of toner than that of a single color image to more firmly be fixed.
Moreover, the strain relief effect of the elastic layer can improve releasability of multicolor toner which tends to be easily offset compared to a single color image. More specifically, the elastic layer compressed and strained at the fuser nip is released from the strain at the exit of the fuser nip, causing misalignment between the elastic layer and toner image at the exit of the fuser nip. As a result, the adhering force on the elastic layer to the toner image is reduced, allowing the image to easily be released from the layer. Furthermore, as the nip shape of the fuser roller and pressure roller at the fuser nip comes to have a convex shape toward the fuser roller side (inverse nip shape), the recording medium can more easily be stripped off from the fuser roller. As a stripping means for separating the fuser roller from the recording medium, self stripping can be realized which allows the recording medium to be separated from the fuser roller without the use of, for example, a strip claw. Therefore, an image failure due to the stripping means can be resolved.
In order to accommodate a higher speed, it is necessary to increase the width of the fuser nip (hereinafter referred to as “fuser nip width”). For increasing the fuser nip width, two ways are listed as examples, including thickening the elastic layer of the fuser roller and increasing the diameter of the fuser roller. As the elastic layer of the fuser roller has a very low thermal conductivity, a thicker elastic layer would increase the warm-up time when a heating means is provided inside the fuser roller. If the processing speed is increased, the temperature of the fuser roller will not follow the fusing temperature. In addition, an increased diameter of the fuser roller would increase the thermal capacity, adversely increasing the power consumption by the heating means.
International Patent Publication WO99/00713 discloses a fusing device for accommodating the increased speed while reducing the warm-up time and power consumption. The fusing device disclosed in WO99/00713 includes a fuser roller, a pressure roller, a heating roller and an endless belt, which uses a belt fixing method in which the endless belt is stretched over the fuser roller and heating roller to make the fuser roller and pressure roller press each other through the endless belt. Since the fusing device does not need a heating means installed in the fuser roller, an elastic layer at the fuser roller may be made thicker and the diameter of the fuser roller is made larger in order to accommodate the increased speed. In addition, the heating roller containing a heater inside can heat the endless belt which has a small thermal capacity, eliminating the need for heating the elastic layer which has a large thermal capacity. This can shorten the warm-up time and avoid increase in power consumption.
Furthermore, Japanese Patent Application Laid-Open No. 02-143274 proposes a fusing device including a fuser head containing a planar heat generator as a heating means, which is arranged opposite from a pressure roller, the heat from the heating means being applied through the fuser belt to a recording medium which is to be heated, to fix a to-be-fixed toner image on the recording medium. The fusing device disclosed in Japanese Patent Application Laid-Open No. 02-143274 can reduce the thermal capacity of the planar heat generator compared to the thermal capacity in the case where a halogen heater is used as the heating means, reducing the power consumption and warm-up time.